DPT immunization: Immunization to protect against diphtheria, pertussis (whooping cough), and tetanus. DPT immunizations are given in a series of 5 shots at 2, 4, 6, 18 months of age and 4-6 years of age. Thanks to vaccination programs, these diseases have become less common. However, there are still unvaccinated individuals capable of carrying and passing diphtheria and pertussis to others who are not vaccinated. Tetanus bacteria are prevalent in natural surroundings, such as contaminated soil. See also: DTaP immunization.